Specify in docs that join='outer' is the defaul for align method.

Author: lexual

doc/source/basics.rst

@@ -869,7 +869,7 @@ Aligning objects with each other with ``align``
 The ``align`` method is the fastest way to simultaneously align two objects. It
 supports a ``join`` argument (related to :ref:`joining and merging <merging>`):
 
-  - ``join='outer'``: take the union of the indexes
+  - ``join='outer'``: take the union of the indexes (default)
   - ``join='left'``: use the calling object's index
   - ``join='right'``: use the passed object's index
   - ``join='inner'``: intersect the indexes